[Lowfer] LF Coil Form

Pat Bunn pbunn at matrixei.com
Mon Feb 10 20:05:07 EST 2014 

I have never seen a rug tube large enough for a typical 74 KHz coil without getting out out of the optimal L vs D range. 

Skin effect make a very large difference at LF. It all depends on what you are willing to pay for, but Litz wire vs THHN on large inductors results in improvements in Q of 200 percent or more. With radiation resistances as low as they are at 137 and 74 KHz, there isn't much you can do to significantly improve it. That kind of Q increase is significant but the wire is going to cost $400 vs $120 for # 12 equivalent Litz wire.

My experience at 74 KHz is MUCH different than at 137 KHz. My 137 KHz antenna loading coil worked with no problems the first time at 400 watts.

At 74 KHz three attempts with added inductors in series with the 137 KHz coil have failed miserably at less than 100 watts.

I have not decided to use Litz wire and probably won't due to cost but if I could find some at a decent cost I would do so. 

Pat
WG2XLPu

Sent from my iPhone

On Feb 10, 2014, at 6:42 PM, Ed Phillips <evp at pacbell.net> wrote:

> Wire size is far more important than coil form and it's cheaper to build a larger coil with bigger wire than mess with getting Litz and using it.  I've had very good luck with plain wire wrapped on a shipping tube for rugs.  Usually peel it from the inside to reduce thickness and give several good coats of shellac.
> 
> Ed
> 
> Pat Bunn wrote:
> 
>> There is a good study on the net comparing various wire and forms. Litz wire ( # 14 eqv) on varnished wood forms for 2 mH inductors had Qs above 600 and the same coil with PVC and THHN wire had Qs on the 200s. That a substantial  difference .
>> 
>> I have had some experience with Sonotube  as telescope tubing. It is loaded with wax which is difficult to remove. It must be scoured off with solvent which is messy. I suspect that it has very high moisture content. The Tesla coil makers state that to keep it from arcing, you must get the wax off and then bake it to get the water out, and then varnish it. I have arcing problems that are bad enough at 74 KHz with PVC with voltages approaching 25 KV.. I'd not take a chance with Sonotube. Too much work to do and then fail. Winding a 200 turn 18 inch diameter coil with #12 AWG THHN is lots of work . 
>> 
>> Pat
>> N4LTA
>> 
>> Sent from my iPhone
>> 
>> On Feb 9, 2014, at 8:33 PM, Ed Phillips <evp at pacbell.net> wrote:
>> 
>> 
>>> I'm a TC guy and never had the PVC pipe problem some guys report.  Find for coil forms in my experience, all the way from a 181 kHz loading coil up through 30 MHz receiver coils.  I suspect Sonotube would be equally good.
>>> 
>>> Ed
>>> 
>>> Pat Bunn wrote:
>>> 
>>>   
>>>> The Tesla coil guys don't like them because the arc. The only way to dry them out is to strip them of the wax impregnation and then dry them and then use electrical varnish in multiple coats. PVC tube is lighter and easier to work with.  Some say varnished wood is better than PVC.
>>>> 
>>>> Pat
>>>> N4LTA
>>>> 
>>>> Sent from my iPhone
>>>> 
>>>> On Feb 9, 2014, at 7:54 PM, Michael Sapp <wa3tts at verizon.net> wrote:
>>>> 
>>>> 
>>>>     
>>>>> Stan & All:  Those compressed cardboard "sonotubes" are popular for telescope building. Might be some useful
>>>>> info in the home scope builder domain for resin coating them.... that's all nasty chemicals tho, for $100 or so it's worth finding a tube section ready to go.... Mike wa3tts
>>>>> 
>>>>> 
>>>>> ----- Original Message ----- From: "stan, W1LE" <stanw1le at verizon.net>
>>>>> To: "Discussion of the Lowfer (US, European, &amp; UK) and MedFer bands" <lowfer at mailman.qth.net>
>>>>> Sent: Sunday, February 09, 2014 6:26 PM
>>>>> Subject: Re: [Lowfer] LF Coil Form
>>>>> 
>>>>> 
>>>>>        
>>>>>> The round concrete form for footings is a good alternative from Home Depot.
>>>>>> Mine is 12.5" OD with 1/8" wall.   Buy the length you need in 4' increments.
>>>>>> 
>>>>>> It looks like compressed fiberboard, but I know that it disintegrates out in the weather,
>>>>>> but it matters not, because by then the concrete is hard.
>>>>>> 
>>>>>> My thought is to wind the coil, then give it a coat or fiberglass resin for a little waterproofing.
>>>>>> 
>>>>>> Other possibilities are a 55 gallon white plastic drum with straight sides or the 190 liter pickle barrels.
>>>>>> I would avoid any colored barrels, like black, because of the pigmentation may effect electrical performance, maybe.
>>>>>> These UN rated plastic barrels are very durable and have lasted for years in their outside storage role.
>>>>>> 
>>>>>> I got my 55 gallon barrels for yard waste from a used barrel dealer.
>>>>>> 
>>>>>> Stan, W1LE     Cape Cod
>>>>>> 
>>>>>> 
>>>>>> 
>>>>>> On 09-Feb-14 4:08 PM, Peter Barick wrote:
>>>>>>            
>>>>>>> OUCH, Old boy! Only $100?? ;~)
>>>>>>> Locally I've picked up scrap green sewer pipe from street work around town (only after hours). Always have remnants of those jobs. Last one was about 5 ft by 18 inches. Also found a 3 foot D pvc drum (no metal rings) that a business discarded full of trash. I emptied it for them. And I have a smaller 18 inch white pvc
>>>>>>> bucket w/ straight sides, another discard.
>>>>>>> Has anyone noticed the cement formers sold in home inprove stores? They are hardened fiber and I believe up to 2 feet in D. And scant few bucks!
>>>>>>> Cheers, Peter, Illinois
>>>>>>> ---------------------------------------
>>>>>>>                
>>>>>>>>>> pbunn <pbunn at matrixei.com> 02/07/14 8:28 AM >>>
>>>>>>> Just a FYI
>>>>>>> 
>>>>>>> I have been searching for quite a while for a reasonably priced coil form for 137 and 75 KHz. It ended up being right under my nose.
>>>>>>> 
>>>>>>> Harrison Superduct - Plastic duct is available in many sizes up to 60" diameter. I just bought a 36" long 16 " OD section for just over 100 bucks. That may sound expensive but compared to Schedule 40 or 80 pipe it is cheap. It is made of PVC and is thin wall duct. They take credit cards and will ship UPS but the plant is about 30 miles from here in Clover, SC and I can pick it up.
>>>>>>> 
>>>>>>> Pat
>>>>>>> WG2XLP
>>>>>>> WG2XCT
>>>>>>> N4LTA
>>>>>>> 
>>>>>>> 
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